AUSTIN, Texas (AP) – Julian Castro was in two places at once. Or so it seemed.

It was April 2005 and there was the baby-faced favorite to win San Antonio's mayoral race perched atop a float chugging down the storied Riverwalk. An announcer even called out his name for 200,000 cheering revelers.

But Castro was right then appearing at a candidates' forum in another part of the city. It turned out the Castro on the float was actually the candidate's twin brother, Joaquin.

The Castros shrugged off the confusion, saying Julian had planned to attend the annual Riverwalk Parade but was delayed. They said they'd not meant to deceive anyone.

"We can't help that we look like each other," Julian Castro told The Associated Press then.

Still, a "Twingate" flap erupted and Castro narrowly lost that summer's mayor's race to a candidate four decades his senior — a rare misstep that kept his meteoric Democratic political rise from happening even faster. Four years later, he was elected San Antonio's mayor and, in 2014, Castro became the youngest member of President Barack Obama's Cabinet as secretary of Housing and Urban Development.

Today, Castro is a possible Hillary Clinton vice presidential pick who could boost Hispanic turnout for the Democratic ticket. But despite the setback of the 2005 electoral defeat, some wonder whether political stardom came too quickly for him — questioning if Castro has enough experience to be Clinton's running mate.

"That's a concern," said Carroll Schubert, a former San Antonio City Council member who ran against Julian Castro in the crowded 2005 mayor's race. "The political resume is thin. That's not to say there haven't been others who served as vice president, even president, with less."

The nation's youngest vice president was John C. Breckinridge, who was 37 when he took office under James Buchanan in 1857. Castro would be 42 when it's time to assume the vice presidency next January. By contrast, Obama's running mate, Joe Biden, served 36 years in Congress before becoming vice president at age 66.

A graduate of Stanford University and Harvard Law School, Castro was working as an attorney when he got a large referral fee in a drunken driving crash from prominent personal injury attorney and top Democratic donor Mikal Watts. The money helped fund Castro's political career. Last year, Watts was indicted on federal fraud charges related to lawsuits over the 2010 BP oil spill.

Castro was first elected to the San Antonio City Council in 2001 at age 26, and during his tenure as mayor the booming city grew to America's seventh largest. Much of the day-to-day operations, though, fall to the city manager, meaning San Antonio's mayor manages little staff and has relatively few executive responsibilities.

Democratic County Judge Nelson Wolff, a longtime Castro ally, countered that the mayor sets policy and wrangles city council votes. He also argued that Castro's youth was a strength.

As HUD secretary, Castro has helped lower home mortgage insurance premiums and expanded public housing broadband internet access. But progressive groups — some of which backed Clinton presidential rival Bernie Sanders — have for months criticized Castro over his agency's selling of delinquent mortgages to Wall Street after the housing crisis. The agency recently announced changes to its mortgage sale process.

Castro had all wings of his party raving about him in 2012. That's when his well-received keynote speech at the Democratic National Convention highlighted the sacrifices of his grandmother — an orphan from Mexico who came to the U.S. at age 6 — and his mother, a top Mexican-American and civil rights activist in San Antonio in the 1960s and '70s.

Lionel Sosa, a San Antonio-based former adviser who helped President George W. Bush capture 40 percent of the Hispanic vote in 2004's presidential election, said the Castro brothers sought his advice when starting out in politics.

"It was pretty obvious that they had been raised the right way and that they were going to do everything they could to make the most of their talent," Sosa said. "They said in unison, 'I want to be mayor' and I asked, 'Which one?' and they both laughed and said, 'Well, one of us will.' "

Joaquin Castro, a minute younger than Julian, served in the Texas House before being elected to Congress in 2012.

Though he doesn't speak fluent Spanish, Julian Castro could help Clinton boost turnout among Latinos, said Gilberto Hinojosa, the first Hispanic chairman of Texas' Democratic Party.

"Someone like Julian Castro as the candidate, it brings a great deal of pride to Hispanics who will participate more," Hinojosa said. He dismissed the inexperience question.